Blood products for use in blood transfusions include, for example, whole blood products prepared by adding an anticoagulant to blood collected from donors, as well as red blood cell products, platelet products and plasma derivatives which are prepared by separating blood components required by a particular recipient from the whole blood products. The leukocytes contained in these blood products, however, are known to cause side effects (e.g. fever response, transfusion-related acute lung injury) and cytomegalovirus infections, and also act as an alloantigen to induce a recipient to produce antileukocyte antibodies, leading to platelet refractoriness.
In order to prevent these incidents, the so-called leukocyte-depleted blood transfusion, in which a blood product is transfused after the removal of the leukocytes contained in the blood product, has become common. The methods for removing leukocytes from a blood product includes two methods, namely a centrifugation method that utilizes the specific gravity difference between the blood components, and a filtration method that utilizes a porous material as a filtering medium. The filtration method has been widely used because it has high leukocyte removal performance, allows easy operation, and costs low.
Meanwhile, long storage of blood products can cause the leukocytes to produce pyrogenic cytokines during the storage, and can lead to death and breakage of the leukocytes that have engulfed viruses or bacteria, allowing pathogens to be spread in the blood for transfusion. Transfusion of such blood containing viruses or bacteria may cause infection in the recipient. Hence, leukocytes need to be removed before storage of blood products.
For prevention of such side effects related to blood transfusion, the Ministry of Health, Labour and Welfare notified the Japanese Red Cross Society of the draft guidance on prestorage leukocyte reduction (to a number of residual leukocytes of not more than 1×106/product) for all products for transfusion in 2003. Then leukocyte depletion filters started to be used for platelet products from donated platelets in 2004, for plasma derivatives from donated plasma in 2005, and for red blood cell products from whole blood in 2006.
Currently used leukocyte depletion filters have leukocyte removal performance (−Log 4) enough to give a number of residual leukocytes of not more than 1×105/product (Non-Patent Literature 1). Still, there is a market demand for filters having higher leukocyte removal performance than the currently used filters so that critical side effects that can be caused by leukocytes transfused into a patient can be completely prevented.
To prevent such side effects, the amount of a filter material to be used has been increased. With the conventional leukocyte depletion filters, however, it is difficult to achieve high leukocyte removal performance enough to completely prevent side effects when the amount of a filter material to be used is just increased.
The following methods for solving such a problem by using a nonwoven fabric as the filter material are known in the art: a method of reducing the average fiber diameter, a method of increasing the pack density, and a method of employing a nonwoven fabric with a more uniform fiber diameter distribution (Patent Literature 1). If these methods are actually employed to enhance the performance so as to give a number of residual leukocytes of not more than 1×104/product, the pressure loss in the filter part when a blood product passes through the filter is increased as the leukocyte removal performance is enhanced. As a result, unfortunately, the blood processing speed extremely decreases before completion of processing for the expected amount of blood.
Moreover, in many cases, the following approaches for blood filtration for prestorage leukocyte reduction are employed in parallel: room-temperature filtration which involves filtering at room temperature within one day after blood collection, and cold-storage filtration which involves filtering after storage for about one to three days in a refrigerator. The cold-storage filtration tends to take more time for filtration. Therefore, leukocyte depletion filters are required to have shorter time for filtration in cold-storage filtration.